The present invention relates generally to optical lithographic techniques commonly used in the formation of integrated circuits and structures on a semiconductor substrate. In particular, the present invention relates to methods of correcting a mask for use in photolithography, systems to perform the correction and apparatus produced from such a corrected mask.
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. The following notice applies to the software and data as described below and in the drawing hereto: Copyright(copyright) 1998, Micron Technology, Inc., All Rights Reserved.
Semiconductor device features are primarily fabricated using photolithography. The art of photolithography embodies techniques for creating two-dimensional patterns on a work surface, or target, by the controlled application of energy (such as electromagnetic, ion beam or other radiation) to a reactive material, or resist, deposited on the target. In a photolithographic process the energy application is controlled through the use of a patterned photomask. The pattern is transferred to a resist coating on the target, forming a resist pattern. The target is then etched according to the resist pattern and, following the etch, subjected to further processing steps. In semiconductor fabrication, the target may be a semiconductor wafer and the resulting features form a portion of a final integrated circuit.
Typically, photolithography is achieved by projecting or transmitting energy through a pattern made of opaque areas and clear areas on a mask. In the case of optical photolithography, the opaque areas of the pattern block light, thereby casting shadows and creating dark areas, while the clear areas allow light to pass, thereby creating light areas. Energy is projected through the clear areas onto and through a lens and subsequently onto the target, such as a semiconductor wafer. The term opaque refers to any area that blocks a sufficient level of the projected energy such that any energy passing through the opaque area will produce only negligible reaction with the resist coating. The term clear refers to any area that permits a sufficient level of energy to project onto the target to react with the resist coating to produce a resist pattern.
In the process of forming a pattern by a projection exposure, it is customary that a member used for reduced-size projection is termed a reticle, and a member for life-size projection is termed a mask; or a member corresponding to an original sheet is termed a reticle, and a member obtained by duplicating such a reticle is termed a mask. In the present invention, any of the masks and reticles classified by such various definitions are referred to as a mask for convenience. Furthermore, the term mask may also refer to a database representation used to produce a physical mask.
The process of producing a mask for an integrated circuit involves generating a composite drawing of the integrated circuit derived from a circuit layout, which is generated from the functional and schematic diagrams. The composite drawing represents the various layers of the integrated circuit, and each layer of the composite drawing will be used to generate a single mask. To transform a layer of the composite drawing into a mask, it is digitized. The resulting database representation defines the opaque and clear areas of the mask. The physical mask is typically produced by selectively establishing areas of opaque material, often a layer of chrome, on a clear support, often a glass or quartz plate. As will be apparent to the reader, areas of the clear support not covered by the opaque material are necessarily clear.
Because of increased semiconductor device complexity which results in increased pattern complexity, and increased pattern packing density on the mask, it is becoming increasingly difficult to produce a precise pattern image despite advances in photolithographic techniques. One problem leading to increased difficulty in transferring a pattern from a mask to the target is proximity effect. Proximity effect can have a pronounced effect when attempting to produce pattern images in the sub-0.5 xcexcm range. Proximity effect describes the distortion of a pattern image resulting from the presence or absence of other optically opaque areas in the immediate proximity of the pattern. An opaque pattern surrounded by large clear areas will produce a smaller image than if the same pattern is surrounded by other opaque areas. Computer programs have been used to purposefully distort the mask pattern to compensate for proximity effect in an attempt to produce a desired image on the target.
Despite corrections for proximity effect, features will still experience distortion due to varying optics and illumination systems of conventional lithography tools. Furthermore, a given lithography tool will inherently produce distortion due to varying intensity levels across its field of exposure. Intensity levels will vary across a field of exposure due to inabilities to achieve even distribution from the energy source as well as inherent imperfections, defects and other characteristics of the optics.
As can be seen, the accuracy of the mask pattern and the resulting resist pattern play important roles in the quality of the circuit. As feature size decreases, the impact of distortion effects increases proportionately. As manufacturing requirements call for exposure of patterns with smaller and smaller dimensions, it is becoming necessary to employ techniques which permit enhancement of the current performance of the process of photolithography.
A method is disclosed wherein a photolithographic mask is corrected based on intensity models for various zones across a field. A variety of semiconductor circuits, dies, modules and electronic systems may be produced from masks produced in accordance with the invention. Such apparatus exhibit improved uniformity of features at the circuit level of the apparatus due to a decrease in distortion not possible through typical proximity effect correction.
In one embodiment, the invention provides a method of correcting a photolithographic mask involving defining at least two zones within a field of the mask, defining intensity models for each of the zones, and modifying features on the mask based on the intensity model for the zone in which the feature occurs. In a further embodiment, the invention provides photolithographic masks wherein features on the mask exhibit corrections based on zones within a field of the mask.
In another embodiment, the invention provides a method of producing a photolithographic mask involving generating a database representation of a physical mask; correcting the database representation by defining at least two zones within a field of the mask, defining intensity models for each of the zones, and modifying features of the mask based on the intensity model for the zone in which the feature occurs; and selectively establishing opaque areas on a clear support in response to the corrected database representation. Selectively establishing opaque areas includes selectively depositing opaque material on a portion of a support, as well as depositing a layer of opaque material on a support and selectively removing portions of that layer. In still another embodiment, the invention provides a photolithographic mask having clear and opaque areas on a clear support, wherein the clear and opaque areas are defined by a process including defining at least two zones within a field of the mask, defining intensity models for each of the zones, and modifying dimensions of the opaque areas of the mask based on the intensity model for the zone in which the opaque area occurs. It should be noted that proximity effect correction alternatively could be applied to clear areas instead of opaque areas with identical results. Furthermore, it is trivial that procedures to modify dimensions of opaque areas necessarily modify dimensions of clear areas and vice versa.
In one embodiment, the invention provides a semiconductor die having circuit patterns produced using photolithographic masks wherein features on the mask exhibit corrections based on zones within a field of the mask. In another embodiment, the invention provides a circuit module, wherein the circuit module contains circuit patterns produced using photolithographic masks wherein features on the mask exhibit corrections based on zones within a field of the mask. In a further embodiment, the invention provides an electronic system having at least one circuit module, wherein the circuit module contains circuit patterns produced using photolithographic masks wherein features on the mask exhibit corrections based on zones within a field of the mask.
In another embodiment, the invention provides a computer program for correcting pattern features for proximity effect based on the intensity variations across the field of exposure. In a further embodiment, the invention provides a machine readable medium having instructions stored thereon for correction of pattern features for proximity effect based on the intensity variations across the field of exposure. In a still further embodiment, the invention provides a system for correcting pattern features for proximity effect based on the intensity variations across the field of exposure, wherein the system has such a machine readable medium.